1. Field of Invention
The present invention relates generally to a method of dry etching semiconductor wafers. More specifically, the invention relates to a method of etching silicon using a gas mixture comprising chlorine with sulfur dioxide.
2. Description of the Background Art
During state-of-the-art semiconductor processing of semiconductor devices, many devices are made in a single substrate, such as a silicon wafer. These devices are connected to each other by means of conductive lines. However, since these conductive lines can introduce unwanted electrical signals in the semiconductor substrate during operation of these devices, the devices are separated from each other by some means of isolation. The usual means of isolation is to etch trenches in the substrate between the devices that can be filled with a dielectric material, such as silicon dioxide.
Such trenches, since they are later filled with a dielectric material, should have sidewalls that are straight and smooth to avoid the formation of voids after filling. The bottom of the trench should have a smooth rounded corner between the sidewall and the bottom as well. Further, it is desirable to be able to etch the trench at a reasonably high etch rate.
Traditionally, trenches are formed in silicon using an anisotropic chemical or reactive ion etch process upon a masked silicon substrate. Chemistries for etching silicon typically utilize combinations of such chemicals as hydrogen bromide (HBr), chlorine (Cl2), oxygen (O2), nitrogen fluoride (NF3), sulfur hexafluoride (SF6), and nitrogen (N2).
For example, using a Cl2 only chemistry can provide a silicon etch rate of about 2853 A/min with a photoresist selectivity (defined as the ratio of the etch rate of silicon to that of the photoresist, or mask, material) of 0.95. A selectivity of less than 1.0 is considered very poor.
Selectivity is one of the limiting criteria when attempting to obtain accurate pattern transfer and small feature sizes on a semiconductor substrate. Generally, higher selectivity allows for smaller feature sizes. To improve wafer processing rates, it is desirable to maximize the silicon etch rate, especially for applications such as trench etching. In general, however, an increase in the silicon etch rate decreases the photoresist etch selectivity.
One method known in the art to improve etch characteristics when using a Cl2 chemistry is to add O2. Such a Cl2/O2 chemistry provides a silicon etch rate of about 3852 A/min with a photoresist selectivity of 1.5. Although the addition of O2 to the Cl2 chemistry yields an increased etch rate and higher selectivity as compared to that of the Cl2 only chemistry, the overall selectivity to the photoresist still remains relatively low.
Therefore, there is a need in the art for a silicon etching process having an increased silicon etch rate while enhancing mask selectivity.
The disadvantages associated with the prior art are overcome by the present invention for etching silicon using a gas mixture comprising gases containing chlorine and sulfur dioxide. In one preferred embodiment, an etch gas (or mixture) comprising chlorine (Cl2) and sulfur dioxide (SO2), is used for etching a silicon substrate at gas flow rates of about 90 sccm Cl2 and about 15 sccm SO2, at a total chamber pressure of about 40 mTorr.
A decoupled plasma source etch reactor is illustratively used to practice one embodiment of the present invention. In general, the reactor uses an inductive source power of about 200-2500 W for plasma generation, and applies a cathode bias power of about 200-300 W to a wafer support pedestal. The reactor maintains the pedestal within a temperature range of about xe2x88x9250 to 100 degrees Celsius. The invention can be practiced, for example, by supplying to the reactor a combination of about 20-300 sccm of chlorine gas and about 2-100 sccm of sulfur dioxide gas, while maintaining a total chamber pressure of about 2-100 mTorr. The gas mixture is supplied to the reaction chamber wherein a plasma is formed and a silicon layer is etched.